The invention relates generally to steam turbines and more specifically to a support structure for a low-pressure steam turbine.
The outer shell of a steam turbine low-pressure section is generally called the exhaust hood. The primary function of an exhaust hood is to divert the steam from the last stage bucket of an inner shell to the condenser with minimal pressure loss. Usually the lower half of the exhaust hood supports an inner casing of the steam turbine and also acts as a supporting structure for the rotor. The upper exhaust hood is usually a cover to guide the steam to the lower half of the hood. The hood for large double-flow low-pressure steam turbines is of substantial dimensions and weight and usually is assembled only in the field. In many steam turbines, the inner case of the steam turbine, for example a double flow/down exhaust unit has an encompassing exhaust hood split vertically and extending along opposite sides and ends of the turbine. This large, box-like structure houses the entire low-pressure section of the turbine. The exhaust steam outlet from the turbine is generally conically-shaped and the steam exhaust is redirected from a generally axial extending flow direction to a flow direction 90 degrees relative to the axial flow direction. This 90-degree flow direction may be in any plane, downwardly, upwardly or transversely. Thus the exhaust hoods for steam turbines constitute a large rectilinear structure at the exit end of the conical section for turning and diffusing the steam flow at right angles.
The lower half of the exhaust hood, split horizontally from the upper half, directs the exhaust flow of steam to a condenser usually located generally beneath the exhaust hood. The lower exhaust hood typically supports the inner casing of the turbine and the associated steam path parts such as diaphragms and the like. The lower exhaust hood is further loaded by an external pressure gradient between atmospheric pressure on the outside and near-vacuum conditions internally. The lower exhaust hood shell is generally of fabricated construction with carbon-steel plates. Typical sidewalls for the lower exhaust hood are flat and vertically oriented. To provide resistance to the inward deflection of the sidewalls under vacuum loading, the lower exhaust hood traditionally has included internal transverse and longitudinal plates and struts. These internal transverse and longitudinal plates and struts form a web, generally underneath the turbine casing and extending to the sidewalls.
FIG. 1 illustrates typical arrangements of a low-pressure double-flow steam turbine 5 with an exhaust hood. An exhaust hood 10 includes an upper exhaust hood 15 and a lower exhaust hood 20, mating at a horizontal joint 22. An inner casing 25 is supported at multiple supporting pads (not shown) on the lower exhaust hood 20. To distribute the load from these pads to an external foundation (not shown) for the low-pressure turbine, various supporting structures are present in the form of transverse plates 35 and struts 40. These transverse plates 35 avoid the suction effect of the sidewalls 45 and end walls 50 and they distribute the load applied on the hood due to loads on inner casing 25. The lower exhaust hood 20 further provides a support location for shaft seals (not shown) and end bearings (not shown) for the turbine rotor (not shown). The lower exhaust hood may include a framework 70 including support ledges 75 that may rest on the external foundation. The sidewalls 45 and end walls 50 may be constructed of flat metal plates, joined at seams by welding or other known joining methods. Steam inlets 30 penetrate each transverse side of the exhaust hood 10. Bearing housings GO for the turbine rotor (not shown) are provided at axial ends of the exhaust hood 10.
The internal hood stiffeners and flow plates are costly. Further, the thick-walled plate used for the sidewalls is also costly. Prior attempts to stiffen exhaust hoods have focused on different combinations of internal stiffeners (pipe struts, plates) and wall thicknesses so as to avoid excess deflection. The problem is that to control the side and end wall deflections of the hood, transverse plates and stiffeners are required inside of the hood. The existence of these transverse and struts increases the complexity of the hood, increases the weight of the hood and creates aero blockages resulting in aero performance losses.
Another distinct adverse impact of the conventional arrangement is the effect of vacuum within the exhaust hood on the steam turbine operation. A vacuum is, of course, required in the operation of a low-pressure steam turbine to extract maximum work from the unit. However, in a conventional exhaust hood, the bearings are located in the cone areas and the inner casing supports are located inside the lower hood. When the exhaust hood is under vacuum, the inner walls and end cones deflect causing misalignment of the steam path rotor parts, end packing and bearing movements/tilt. The extended walls of the lower exhaust hood also support the inner exhaust casing in the conventional arrangement. The extended walls include hood footplates and supporting gussets. The height of the extended wall may be nearly 5 feet. Temperature and pressure changes in the hood will alter the position of the inner casing being supported by the hood wall, thereby impacting clearances of the rotor relative to the end bearings and the leakage labyrinths.
Accordingly, it would be desirable to provide a support structure for a low-pressure steam turbine that reduces operating misalignment between the rotor and the stationary members and at the same time reduce structural complexity, cost, and obstruction to aerodynamic performance.